I was talking about the exam 
The amp is sounding good, except for that annoing cracking/frying sound from the left channel. I'm starting to think that's ground related, since I've returned the grounds of the silent channel directly on the soldering tab of the wire of the minus side of the last filter cap, but the left channel is grounded in another tab which is connected to the main one by a wire... I've changed the incriminated potentiometer with no luck. Also changing the tube doesn't help: it always stays in the left channel. No arcing, smoking, or other strange things: in fact, the 1/4W resistors I'm using are holding up well (I'm talking about cathode and gridstoppers ones, not the big one in the psu!).
Some residual hum, anyway, but it's expected when on a wooden test board, with unshielded input and output wires.
About the exam... well, we've got a very very bad math teacher, but if I start talking we'll go offtopic
The amp is sounding good, except for that annoing cracking/frying sound from the left channel. I'm starting to think that's ground related, since I've returned the grounds of the silent channel directly on the soldering tab of the wire of the minus side of the last filter cap, but the left channel is grounded in another tab which is connected to the main one by a wire... I've changed the incriminated potentiometer with no luck. Also changing the tube doesn't help: it always stays in the left channel. No arcing, smoking, or other strange things: in fact, the 1/4W resistors I'm using are holding up well (I'm talking about cathode and gridstoppers ones, not the big one in the psu!).
Some residual hum, anyway, but it's expected when on a wooden test board, with unshielded input and output wires.
About the exam... well, we've got a very very bad math teacher, but if I start talking we'll go offtopic
Oh yes, I've tried many new tubes.
Someone pointed me that the occasional popping could be related to excessive h-k voltage. I had the heaters elevated above ground by 35V, from a resistive voltage divider from the B+. I tried to reduce this to 10V, but now I've got a strong wooshing sound (like water, big time hiss, almost white noise) at startup, like 10/20 seconds when the 220uF cap in the heater reference circuit is charging, and after it has reached the proper 10V voltage, the noise stops and there's a faint buzz, like rectifier noise. I've also tried to put the heaters directly to ground, with the same results. Btw, on the buzz there is also a random and faint popping, like before.
I'm doing more research, now I will retry all the tubes I've got... I'll let you know!
Someone pointed me that the occasional popping could be related to excessive h-k voltage. I had the heaters elevated above ground by 35V, from a resistive voltage divider from the B+. I tried to reduce this to 10V, but now I've got a strong wooshing sound (like water, big time hiss, almost white noise) at startup, like 10/20 seconds when the 220uF cap in the heater reference circuit is charging, and after it has reached the proper 10V voltage, the noise stops and there's a faint buzz, like rectifier noise. I've also tried to put the heaters directly to ground, with the same results. Btw, on the buzz there is also a random and faint popping, like before.
I'm doing more research, now I will retry all the tubes I've got... I'll let you know!
Hi Giaime,
Most tubes are good for 100VDC between the heater and cathode, 200V peak (AC + DC components). I normally run my heaters around 30 VDC positive to ground. That puts most cathodes 28~29 VDC above ground. I can not foresee any problems due to that.
Are your sockets new? It is possible to get a track between pins and an older socket may have developed one. You would then inherit the problem from the old equipment.
-Chris
Most tubes are good for 100VDC between the heater and cathode, 200V peak (AC + DC components). I normally run my heaters around 30 VDC positive to ground. That puts most cathodes 28~29 VDC above ground. I can not foresee any problems due to that.
Are your sockets new? It is possible to get a track between pins and an older socket may have developed one. You would then inherit the problem from the old equipment.
-Chris
Yes Chris, sockets are old, but I carefully clean them.
I've tried all the tubes I've got, 4 ECC81 by Siemens. Similar date code and identical structure.
2 of them are absolutely quiet, no noise and zero popping. 2 of them make the noise. The interesting thing is that I knew that a lot of tubes flash at startup, normally because cold heaters represent a very low resistance, so big current flows until the heater has reached it's proper temperature. Incidentally, the 2 good tubes don't flash, the 2 bad ones flash. Is this related?
Now I'm going to restore the 35V reference on the heaters, to kill the buzz I'm hearing. Stay tuned...
I've tried all the tubes I've got, 4 ECC81 by Siemens. Similar date code and identical structure.
2 of them are absolutely quiet, no noise and zero popping. 2 of them make the noise. The interesting thing is that I knew that a lot of tubes flash at startup, normally because cold heaters represent a very low resistance, so big current flows until the heater has reached it's proper temperature. Incidentally, the 2 good tubes don't flash, the 2 bad ones flash. Is this related?
Now I'm going to restore the 35V reference on the heaters, to kill the buzz I'm hearing. Stay tuned...
Hi Giaime,
No relation to the flashing. That's only how much of the heater is visable beyond the cathode tube.
A carbon track can be burned into the surface of a tube socket, so cleaning will not always clear that up. You would need to scrape it out and as a safety, use some dope or varnish to seal the area.
Like choky, I only use 10uF on the heater tap. You don't need the time constant a 220uF will create. No need to change it yet, but try a 10uF later to see if you get rid of the declining buzz on power up.
-Chris
No relation to the flashing. That's only how much of the heater is visable beyond the cathode tube.
A carbon track can be burned into the surface of a tube socket, so cleaning will not always clear that up. You would need to scrape it out and as a safety, use some dope or varnish to seal the area.
Like choky, I only use 10uF on the heater tap. You don't need the time constant a 220uF will create. No need to change it yet, but try a 10uF later to see if you get rid of the declining buzz on power up.
-Chris
I've put my heaters back on 35V. I've got little less hum, and no popping/cracking yet. On the heater reference, I put a 47uF 50V, since it's the only hi-voltage cap I've got here in my parts bin. I've got smaller one, but no one rated for 35V or thereabouts. And those are ELNA, so they're cool
But I'm also consicious of the fact that when I change tubes, I put a stress on the socket, thus I could have removed or put an intermittent contact between heather and cathode. Anyway, I'll retry the tubes I've marked as "bad", it's a pity to loose such valuable Siemens tubes. Tubestore quotes them at 30$...
Still some hum: I think I'm done for now, the residual hum is surely due to lack of shielding, tube near the PTs, etc etc... typical prototipe problems. Anyway as said I'm going to design a PCB for it, incorporating that phono preamp.
I've just downloaded Eagle Cad and I'm starting to tinker around...
Thanks to everyone for the help!
But I'm also consicious of the fact that when I change tubes, I put a stress on the socket, thus I could have removed or put an intermittent contact between heather and cathode. Anyway, I'll retry the tubes I've marked as "bad", it's a pity to loose such valuable Siemens tubes. Tubestore quotes them at 30$...
Still some hum: I think I'm done for now, the residual hum is surely due to lack of shielding, tube near the PTs, etc etc... typical prototipe problems. Anyway as said I'm going to design a PCB for it, incorporating that phono preamp.
I've just downloaded Eagle Cad and I'm starting to tinker around...
Thanks to everyone for the help!
Hi Giaime,
I use 50V types for them heater potential filter, but a 35V should be okay. It's not very likely to rise above that unless you pull all the tubes and turn it on.
The tracking occurs when a current flows between pins due to moisture or some deposit. It may eventually burn a track in the socket material. Inserting tubes is unlikely to cause that damage.
Your hum should be next to non-existant, once it's in a metal case. If you design a PCB, plan for ventillation holes around the tubes. Keep traces well away from each other if you can. Consider running the heaters on twisted pair wire off the PCB.
-Chris
I use 50V types for them heater potential filter, but a 35V should be okay. It's not very likely to rise above that unless you pull all the tubes and turn it on.
The tracking occurs when a current flows between pins due to moisture or some deposit. It may eventually burn a track in the socket material. Inserting tubes is unlikely to cause that damage.
Your hum should be next to non-existant, once it's in a metal case. If you design a PCB, plan for ventillation holes around the tubes. Keep traces well away from each other if you can. Consider running the heaters on twisted pair wire off the PCB.
-Chris
anatech said:
Oh yes, in fact I used a 47uF 50V as I said.
anatech said:
Yes, but you haven't seen my construction... the tube socket is hanging on the passive components, not attached to anything with his proper holes. So I thought that maybe I bent some components and made some shorts... but I've double checked, I've identified the tubes as bad, they still make noise. So it was tube related.
anatech said:
Thanks for the suggestions! I was planning on 2 separate PCBs, one for the power supply (including regulated DC filaments and regulated DC B+), and the other one for the audio circuits. So I can run the heater tracks near the audio ones without big time hum (obviously I'll avoid running them near if possible!)
Oh man, how good this preamp sounds. It really opened up the (a little anemic) Marantz power amp I'm using, and the cheap JVC speakers.
But I'm going to collect some ideas for a definitive schematic.
I was suggested by a local hi-end guru that the CF arrangement is bad, due to the 100% NFB that it introduces. I'm not totally against NFB, but I'd like to avoid id. So let's put some ideas:
I need a preamp that does:
- accept a common MM phono input
- accept a normal line input
- provide also some gain
- have some way to get a low output Z
Preferred realizations of those ideas:
- the phono pre will follow the already posted schematic, tweaked to accept an ECC83 in the second stage.
- since I'm planning to drive a tube power amp, I don't need exceptionally low output Z. But I'd like to have it very low: I thought of a common cathode output stage with an ECC81, that has little gain and lower Z than a common ECC83. I also thought of paralleling the two sections.
- maybe a selectable cathode / plate output, so with the filp of a switch I can have either gain with high output Z (when I'm driving tube amps) or no line gain with low output Z (ex. long cables, SS amps...)
- avoid caps in the signal path. I'm hoping to make direct coupled stages where possible.
- PSU: no chokes please, they're heavy and expensive. But if it can help, I still have that fluorescent lamp choke. I'd like a SS rectified and SS regulated B+ (for this, I have many power BTJs and Mosfets here, so any schematics and suggestions will be helpful. What about switching mosfets from old TVs and pc smps? I have many...).
Filaments will be SS rectified, cap bank and LM317 to take them to 6.3V (I'm not sure about this, I've seen many schematics using those voltage regulators, and I have some of them in my parts bin).
The construction probably will be on PCB, 2 ones: one for the psu, and another for the audio circuits.
Iron that I have: a 110V no CT @ 25VA tranny, and a multi-voltage 2A tranny for filaments (with many low voltage secondaries).
Suggestions / Hints?
But I'm going to collect some ideas for a definitive schematic.
I was suggested by a local hi-end guru that the CF arrangement is bad, due to the 100% NFB that it introduces. I'm not totally against NFB, but I'd like to avoid id. So let's put some ideas:
I need a preamp that does:
- accept a common MM phono input
- accept a normal line input
- provide also some gain
- have some way to get a low output Z
Preferred realizations of those ideas:
- the phono pre will follow the already posted schematic, tweaked to accept an ECC83 in the second stage.
- since I'm planning to drive a tube power amp, I don't need exceptionally low output Z. But I'd like to have it very low: I thought of a common cathode output stage with an ECC81, that has little gain and lower Z than a common ECC83. I also thought of paralleling the two sections.
- maybe a selectable cathode / plate output, so with the filp of a switch I can have either gain with high output Z (when I'm driving tube amps) or no line gain with low output Z (ex. long cables, SS amps...)
- avoid caps in the signal path. I'm hoping to make direct coupled stages where possible.
- PSU: no chokes please, they're heavy and expensive. But if it can help, I still have that fluorescent lamp choke. I'd like a SS rectified and SS regulated B+ (for this, I have many power BTJs and Mosfets here, so any schematics and suggestions will be helpful. What about switching mosfets from old TVs and pc smps? I have many...).
Filaments will be SS rectified, cap bank and LM317 to take them to 6.3V (I'm not sure about this, I've seen many schematics using those voltage regulators, and I have some of them in my parts bin).
The construction probably will be on PCB, 2 ones: one for the psu, and another for the audio circuits.
Iron that I have: a 110V no CT @ 25VA tranny, and a multi-voltage 2A tranny for filaments (with many low voltage secondaries).
Suggestions / Hints?
Hi Giaime,
Cathode followers are much better than trying to drive the cable and following circuit with a high impedance plate. Also your eq network for the phono stage. The feedback is local, not global, so those comments are baseless. A CF is better than a transformer in my book, so what are you going to do?
-Chris
Cathode followers are much better than trying to drive the cable and following circuit with a high impedance plate. Also your eq network for the phono stage. The feedback is local, not global, so those comments are baseless. A CF is better than a transformer in my book, so what are you going to do?
-Chris
NFB should not be treated as a bad. The whole mechanized world operates because of NFB... only in audio is it perceived as bad.
This is because several bad amplifiers have been treated with large amounts of GLOBAL (around the whole amplifier) feedback in an effort to make them good.
If the bandwidth of a good (linear) amplifier is 10Hz - 100kHz then Global feedback in careful amounts will make it perform better still.
LOCAL feedback, as in the case of a CF, is one type of feedback that helps to make the "good" amp linear.
NFB, all forms, is very difficult to engineer & understand completely. It is usually the last (final) thing engineers are taught in school.
When feedback is FREE and NO engineering is required, like a CF... USE it... it is a gift. Listen to your moderator!
This is because several bad amplifiers have been treated with large amounts of GLOBAL (around the whole amplifier) feedback in an effort to make them good.
If the bandwidth of a good (linear) amplifier is 10Hz - 100kHz then Global feedback in careful amounts will make it perform better still.
LOCAL feedback, as in the case of a CF, is one type of feedback that helps to make the "good" amp linear.
NFB, all forms, is very difficult to engineer & understand completely. It is usually the last (final) thing engineers are taught in school.
When feedback is FREE and NO engineering is required, like a CF... USE it... it is a gift. Listen to your moderator!
Yes I agree with you. It was just an idea, thrown here just to evaluate all possible possibilities.
This is to underline the lack of fundamentals of a kind of hi-end gurus...
But a thing makes me think. Chris said:
What do you mean? How could the CF drive the phono stage if it's the last thing in the preamp?
I mean I was planning to use the CF as the output circuit of the pre, not to drive the phono pre (that would be useless).
So, I'm open for suggestions. Especially on psu regulation etc etc.
I'm searching for options, but the help of someone more experienced can be useful!
If you think that this thread has too limited visibility, I'll try to open a new one for the preamp project.
Thanks to all!
This is to underline the lack of fundamentals of a kind of hi-end gurus...
But a thing makes me think. Chris said:
anatech said:
What do you mean?
How could the CF drive the phono stage if it's the last thing in the preamp?I mean I was planning to use the CF as the output circuit of the pre, not to drive the phono pre (that would be useless).
So, I'm open for suggestions. Especially on psu regulation etc etc.
I'm searching for options, but the help of someone more experienced can be useful!
If you think that this thread has too limited visibility, I'll try to open a new one for the preamp project.
Thanks to all!
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